Magnetic read/write heads in a disk drive system including a magnetic medium are subject to a phenomenon known as "domain pinning" which causes the head to enter a metastable state resulting in unbalanced magnetization. This condition inhibits efficient reading and recovery of pre-recorded data. In particular this problem affects disk drive heads having narrow recording track width and a structure with relatively few magnetic domains.
Typically, if error correction circuitry is unable to appropriately correct errors read, the reading operation can be retried in different ways before ending the operation and recording the error. The algorithm used by the system generally attempts a number of possible methods available (and allowed by the host computer), determined by the type of operation and the cause of the error.
Commonly, the retry algorithm executes, for example, four or five steps, and if the steps are unsuccessful, repeats the same several steps a predetermined number of retries before reporting an error. As an example, in Supino et al., U.S. Pat. No. 5,053,892, if errors are found to be uncorrectable after a fifth attempt at error correction, the block of data is declared uncorrectable.
Using a set number of retries sacrifices the aggressiveness of the recovery algorithm. That is, if more time is available to attempt retries, a more efficient use of that time would be for the retry algorithm to operate during the time available to attempt additional, and perhaps varied retries.
It is especially difficult to mix recovery methods that take different amounts of time.
It is well known that if the magnetic head is temporarily energized in a cyclical write state, it may randomly relax to a stable and symmetrical domain state (i.e., be recovered), which then allows normal reading and data recovery, as described for example in U.S. Pat. No. 5,053,892 to Supino et al.
Supino et al. teach moving the read/write head via a seek command to a remote auxiliary track location which contains no data, and there performing a randomized write operation. Then, a return seek operation is undertaken to find the desired sector to be read, and it is then determined if the head reads the data properly. This procedure requires on the average a period of time equal to 2 times (average seek time plus the average rotational latency time of the disk). Since the process is not stochiastic, the operation may need to be repeated to obtain the desired domain state, resulting in an undesirably long data recovery procedure.
A further problem is that a disk drive utilizing pre-recorded sector information may be unable to decode subsequent sector information when the read/write head enters the undesirable metastable state immediately following a data information write operation. In this condition the disk drive is dysfunctional and diagnostic writing procedures cannot be achieved.
Of more general interest is U.S. Pat. No. 5,121,262 to Squires et al.